Impregnating and coating composition for porous ceramic insulation

ABSTRACT

A COATING AND IMPREGNATING COMPOSITION SUITABLE FOR PROVIDING A WATER REPELLENT, ABRASION, SCRATCH, CHIP AND DIRT RESISTANT COATING ON A CERAMIC INSULATING SURFACE COMPRISING A SOLVENT, A SUSPENSION OF A RESIN, A SILICONE AND A PARTIALLY HYDROLYZED TETRA-ALKYL ORTHOSILICATE.

Uted States ABSTRACT OF THE DISCLOSURE A coating and impregnatingcomposition suitable for providing a water repellent, abrasion, scratch,chip and dirt resistant coating on a ceramic insulating surfacecomprising a solvent, a suspension of a resin, a silicone and apartially hydrolyzed tetra-alkyl orthosilicate.

BACKGROUND OF THE INVENTION In the manufacture of electrical bodies,such as electrically conducting materials, resistors, etc., it has beensuggested to insulate the electrical component With a porous ceramicmaterial. These insulating layers are conveniently formed by powdermetallurgical techniques or any other method suitable for the formationof a ceramic layer. The materials employed as the insulating layer maybe magnesia, alumina, zirconia, beryllia, thoria, etc. or mixturesthereof. Since these insulating materials are porous, it is oftennecessary to provide the outer surface thereof with a sealing layer inorder to prevent moisture absorption in the electrical component and toinsure insulation.

Heretofore, various materials have been suggested for coating andsealing ceramic insulating materials. Thus, Pat. No. 3,032,444 toScadron et al., May 1, 1962, suggests the use of a methyl siliconepolymer as a sealing composition. Although this composition representsan improvement over the then prior art, there are numerous disadvantagesassociated with this and other proposed prior art sealing compositions.Although the prior art sealing compositions effectively reduce theporosity of the ceramic insulator, they have a low abrasion, scratch andchip resistance. As a result, during normal use, portions of the sealinglayer are abraded away, thereby exposing the underlying insulator andelectrical component.

Moreover, many of the proposed prior art sealing compositions are notsufliciently flame-proof to enable their use in high temperatureelectrical applications. In other words, many of the prior art sealantswhen employed in connection with electrical components which aresubjected to high voltages, burn or are subject to external arcingduring operation. In addition, the conventional sealants are notsufliciently resistant to arcing to permit their commercial use.

The prior art is faced with the diflicult problem of providing a sealantcomposition for ceramic insulators which is not only flame-proof andhighly resistant to arcing, but is also scratch, chip and abrasionresistant while providing a high degree of water repellency and beingable to effectively seal a porous ceramic substrate.

BRIEF DESCRIPTION OF THE INVENTION These prior art difliculties areovercome by the present invention. The invention provides a protectiveimpregnant and coating composition for ceramic insulation which ishighly abrasion, scratch and chip resistant, water-repellent, dirtresistant, flame-proof, resistant to external arcing and otherwiseimproves the electrical characteristics of the ceramic insulatingmaterial. Briefly, the composition is a low viscosty liquid having asmall amount of material in suspension comprising:

(a) a partially hydrolyzed tetra-alkyl orthosilicate,

(b) a resin suspension,

(0) a silicone, and

(d) a solvent which is compatible with (a), (b) and (c).

The invention also relates to electrical components having a ceramicinsulation wherein the ceramic insulating material is coated wth theabove composition.

The above coating composition penetrates the ceramic material, exerts abinding action on the individual ceramic particles, fills the surfacepores and deposits an abrasion, scratch and chip resistant glossy orsemi-glossy thin, hard film over the ceramic surface which is highlyflame-proof and resistant to external arcing.

DETAILED DESCRIPTION OF THE INVENTION The partially hydrolyzedtetra-alkyl orthosilicate serves to help fill the individual pores inthe ceramic material and operates as an adhesive, exerting a bindingaction on the individual ceramic particles. It also renders the systemarc and flame resistant and reduces smoking at very high temperatures.Among the suitable partially hydrolyzed tetra-alkyl orthosilicates arethe methyl, ethyl, propyl orthosilicates. The most preferred material isa partially hydrolyzed tetra-ethyl orthosilicate.

The degree of hydrolysis of the partially hydrolyzed tetra-alkylorthosilicate is not overly critical. Generally, however, thosetetra-alkyl orthosilicates having a higher degree of hydrolysis havebeen found to be more preferable. The degree of hydrolysis may rangefrom about 10% to about The silicone component acts as an adhesive andpore filler while improving the surface finish of the resultant coating.More importantly, the silicone component provides a high degree of chipresistance and water repellence. The particular silicones employed arenot overly critical. Any of the well known silicone resins or siliconecompositions including a solvent and curing agent which are compatiblewith the remainder of the ingredients may be employed.

Generally, those silicones prepared from trifunctional silanesrepresented by the following structural formula may be employed:

wherein R is an aliphatic or aromatic substituent. It has been foundthat these silicones combine the best properties of flame and areresistance with flexibility.

Obviously, where flexibility is not important, those silicones preparedfrom tetrafunctional silanes may be used. The silicones made fromdifunctional silanes are, of course, more flexible. It is to be furtherunderstood that mixtures of silicones may be employed to produce aparticular combination of properties.

It is to be understood also that the term silicone is intended toinclude those commercially available materials which include a solventor dispersing agent and catalyst or curing agent.

The resin suspension operates to fill surface pores and, moreimportantly, improves the abrasion and scratch resistance of theresultant coating.

Any resinous material having a low coefficient of friction may beemployed. Where a high degree of flame and are resistance is required,however, it is preferred to employ suspensions of the fluorocarbonresins such as those of tetrafluoroethylene, trifluorochloroethylene,fluorinated ethylene-propylene resins, vinylidene fluoride, copolymersof tetrafluoroethylene and hexafiuoropropylene, vinyl fluoride andtolomers of tetrafluoroethylene.

It is to be understood, however, that where high flame and areresistance are not critical, any suitable resin may be employed.Generally, any resin having a relatively low coefiicient of friction issuitable to improve the abrasion and scratch resistance of the finalcoating. Generally, the coefficient of friction (static) may vary fromabout 0.04 to about 1.00. Suitable non-fluorinated resins, for example,are polyethylene and polypropylene.

The liquid carrier in which the resin is suspended may be any liquid inwhich the resin is insoluble and which is compatible with the otheringredients of the composition. Among the suitable liquids are toluene,benzene, xylene, etc. As is apparent to those skilled in the art, theselection of the particular dispersing liquid will depend on the resinselected and the remaining ingredients in the coating composition.

The solvent component of the coating composition operates to lower theviscosity and reduce the surface tension of the coating composition. Inmany cases, the solvent also operates as a stabilizer. Generally, anyrelatively anhydrous organic solvent which is compatible with theremaining ingredients in the composition may be employed, for example,isopropanol, trichloroethylene, perchloroethylene, n-propanol ethanol,dichloroethylene, isobutanol, butanol. It is only necessary that thesolvent have no sol vating eifect in the resin component.

The amounts of ingredients in the composition are not overly criticaland depend, to a large extent, on the desired properties in the finishedcoating.

The amounts of partially hydrolyzed tetraalkyl orthosilicate may varyfrom about 1% to about 95% by weight. At the lower percentages, theflames and are resistance are flower, thereby decreasing the temperaturestability and flamability resistance of the finished article. However.higher percentages increase surface friction and reduce the chip andscratch resistance of the finished coating. Moreover, dirt resistance islower at the higher percentages. Thus, in those instances wheretemperature stability and fiarnability resistance is not a criticalnecessity, lower percentages of the ester may be employed. In thosesituations where chip and scratch resistance are of secondary importanceand flamability resistance and high temperature stability is arequisite, higher percentages of the ester may be employed. Generally,amounts ranging from about 16% to about 50% by weight provide a goodbalance of temperature stability properties and chip, scratch and dirtresistant properties.

The amount of silicone employed may vary from about 1% to about 90% byWeight. Here, also, the properties desired in the finished coatingdetermine the percentage of silicone employed. At the lowerconcentrations, the chip, scratch and dirt resistance is lowered. At thehigher concentrations, the coating may form a thick layer which issubject to cracking during a curing step where curing is necessary. Alsoat the higher percentages, the flame and are resistance are lowered.Accordingly, where flame and are resistance are not overly critical,higher percentages of silicone may be employed. In those situationswhere chip, scratch and dirt resistance are not necessary, lowpercentages of the silicone ingredient may be employed.

The amounts of resin suspension may vary from about 1% to about 75% byweight. The amount of suspension employed is dependent on two factors.

( 1) the percentage of resin in the suspension, and (2) the amount ofsolvent present in the overall coating composition.

Generally, the amount of resin in the suspension may vary from bout 0.1%to about 52%. Lower percentages of the resin suspension in the overallcoating composition lowers the abrasion resistance. At the higherpercentages, the chip and scratch resistance are increased. Thepreferred 4 percentages of resin suspension in the final coatingcomposition are from about 7% to about 33%.

It will be understood that rather than employing a dispersion of theresin as an ingredient in the coating composition, the resin may bedispersed in the solvent component of the coating composition.

The resin particle size distribution may vary from submicron to aboutmicrons. It is necessary also that the solvent and carrier liquid beselected so that the resin particles remain in finely divided suspensionform in the resulting coating composition. In addition to providing ascratch and abrasion resistant surface, the resin particles also operateto enter the pores of the ceramic insulating material and act as asealant.

The amount of solvent employed may vary from about 1% to about 80% byweight. Large amounts of solvent decrease the chip, scratch and dirtresistance of the final coating but do not lower the coefiicient offriction of the ceramic surface to which the coating composition isapplied. The employment of small amounts of solvent results in a highlyviscous composition which has poor impregnation properties. It ispreferred to employ an amount of solvent ranging from about 10% to about40%.

It is to be understood that by the term solven is meant those liquidswhich are compatible with the partially hydrolyzed tetra-alkylorthosilicate and silicone ingredients of the coating composition andare also compatible with the carrier liquid for the resin suspension,where employed, but does not dissolve the resin itself.

Generally, any conventional coating method may be employed to apply thecoating compositions of the invention to an electrical component. Amongthe available methods may be mentioned spraying, fountain, dip coating,electrostatic spraying, painting, etc.

'It is also to be understood that conventional pigment and filleradditives may also be added to the compositions of the presentinvention. Among these materials may be mentioned inorganic hightemperature pigments such as ferric oxide, cobaltous aluminate andchromium oxide; fillers such as aluminum oxide, silica and mica.

Depending on the particular silicone ingredient employed in the coatingcomposition, it may be necessary to cure the coated article. Thus, thereare available various silicone compositions which are dispersions insolutions of the silicone and a curing agent. It will be necessary toeffect a heat cure of these compositions. Where required, a curingtemperature in the range of from about to about 250 C. may be used.

The method of mixing and the order of addition of the variousingredients of the composition of the present invention are notcritical. Any conventional mixing method may be employed to integratethe materials.

EXAMPLE 1 A coating composition having the following formula wasprepared:

Parts by weight Partially hydrolyzed tetra-ethyl orthosilicate (SilbondH-4, Staulfer Chemical Company) 8 Silicone (S2-4SC), Midland IndustrialFinishes Company) 5 Isopropanol, anhydrous 6 Teflon suspension (MS-142C,Miller-Stephenson Company, 2-3% Teflon in toluene) 5 The abovecomponents were found to be completely compatible with one another andthe overall composition was found to have an indefinite shelf life at 70F.

EXAMPLE 2 Several FP-4, 39K tin oxide resistors were coated with aceramic insulating layer employing a composition comprising aluminumoxide and silicon dioxide fillers, titanium dioxide and cobaltousaluminate pigments, isopropanol and prehydrolyzed tetra-ethylorthosilicate binder. The

coated resistors were heat cured to yield a ceramic coating mils inthickness.

The resulting assemblies were impregnated with the coating compositionof Example 1 and again heat cured.

The resistors were subjected to overloads of 50 X. No evidence ofburning, smoking or arcing was observed. The resistors, rather openedthe circuit. They were found to be highly resistant to chipping,scratching, abrasion and dirt. When immersed in water, the resistors didnot absorb water and when removed from the water, only a few dropletsadhered to the surface, indicating a high degree of water resistance.

Ordinary ceramic coated resistors act as a sponge and absorb largequantities of water when immersed.

The impregnating compositions of the invention find applications infields other than ceramic insulators for electrical components. They maybe employed to protect, any porous ceramic type material, such as theinorganic zinc based coatings employed to coat hulls, tanks, strlrcturalsteel, etc. Generally, any ceramic material may be protected by thecomposition of the present invention.

What is claimed is:

1. A porous ceramic insulation impregnating and coating compositionsuitable for ceramic insulation comprising from about 1% to about 95% ofa partially hydrolyzed tetra-alkyl orthosilicate, from about 1% to about90% of a polysiloxane, from about 1% to about 75% of suspended resinousparticles and from about 1% to about 80% of a solvent, all percentagesbeing parts by weight.

2. The composition of claim 1 wherein said orthosilicate comprises apartially hydrolyzed tetra-ethyl orthosilicate.

3. The composition of claim 1 wherein said silicone is derived from atrifunctional silane.

4. The composition of claim 1 wherein said suspension of resinousparticles comprises a suspension of polytetrafluoroethylene in a carrierliquid.

5. The composition of claim 4 wherein said carrier liquid is an aromaticliquid.

6. The compound of claim 1 wherein said solvent is isopropanol.

7. The composition of claim 1 wherein said orthosilicate is a memberselected from the group consisting of a partialy hydrolyzedtetra-methyl, ethyl or propyl orthosilicate.

8. The composition of claim 1 wherein said orthosilicate is a partiallyhydrolyzed tetra-lower alkyl orthosilicate.

9. The composition of claim 1 wherein said polysiloxane is selected fromthe group consisting of di-, triand tetra-functional silanes.

10. The composition of claim 1 wherein said resinous particles have acoefiicient of friction within the range of from about 0.04 to about1.00.

11. The composition of claim 1 wherein the degree of hydrolysis of saidpartially hydrolyzed tetra-alkyl orthosilicate is from about 10% toabout 90%.

12. A porous ceramic insulation impregnating and coating compositionconsisting essentially of from about 1% to about 95% of a partiallyhydrolyzed tetra-ethyl orthosilicate, from about 1% to about 90% of asilicone derived from a trifunctional silane, from about 1% to about ofa suspension consisting essentially of about 3% ofpolytetrafluoroethylene in toluene, and from about 1% to about ofisopropanol.

13. The porous ceramic insulation impregnating and coating compositionof claim 1 wherein said partially hydrolyzed tetra-alkyl orthosilicatecomprises from about 16 to about 50% of said coating composition, saidsuspended resinous particles comprise from about 7 to about 33% of saidcoating composition, and said solvent comprises from about 10% to about40% of said coating composition, all percentages being parts by weight.

References Cited UNITED STATES PATENTS 3,028,347 4/ 196 2 Chevalier260-33.4(Si) 3,062,764 11/1962 Osdal 260827 3,392,130 7/1968 Rucker etal 260-827 MORRIS LIEBMAN, Primary Examiner L. T. JACOBS, AssistantExaminer U.S. Cl. X.R. 26033.8, 827

53 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORR Patent No.3,575,916 Dated April 20, 1971 Invenwfl Lawrence G. Bockstie, Jr.

It is certified that error appears in the above-identified pate1 andthat said Letters Patent are hereby corrected as shown below:

Column 3, line 35, "flower" should be lower Claim 6, line 1, "compound"should be composition Claim 7, line 3, "partialy" should be partiallySigned and sealed this 15th day of February 1972.

(SEAL) Attest:

EDWARD M.FLETCHER, JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

